Benthic microbial-produced reef structures from the Cambrian and Ordovician often take the form known as thrombolites that are recognized by clotted texture as opposed to thin or thick laminae of classic stromatolites. Generally speaking, stromatolite systematics in the literature is plagued by grossly inconsistent ontology's and terminologies, and presents very recondite. Some authors attribute the of thrombolites to carbonate precipitation induced by bacteria under low sedimentation rate and low water energy when a Paleozoic reef came under ecological stress; their formation, however, is very complex conjecture that is not well understood. Sometimes called a cryptalgal clot, where clot refers to a macro-structural column; the key distinction is the stromatolite lacks distinct lamination.

Keep in mind that Paleozoic reef system were built by a multiplicity of complex animals (e.g., coral) and single cell forms such as Eubacteria using many types of metabolism, as well as the Eukaryotic, photosynthetic algae.

This Ordovician "thrombolite" from the Oneota Formation, Buffalo County, Wisconsin, is very rare in the formation.